1. Field of the Invention
The present invention relates to a broad-band wavelength-tunable semiconductor laser needed in an optical fiber communication technique used for a telephone exchange network of a trunk line or the like, particularly a wavelength division multiplexing system that simultaneously uses laser beams having different wavelengths for signal transmission, and, more particularly, to a wavelength-tunable semiconductor laser having an optical waveguide with a front light reflection area comprising a sampled grating mirror and a rear light reflection area comprising a super-structure-grating mirror that are located before and after an active region, respectively, and an optical module using the wavelength-tunable semiconductor laser as a light source.
2. Description of the Related Art
An SSG-DBR wavelength-tunable semiconductor laser using a so-called SSG (Super-Structure-Grating) DBR (Distributed Bragg Reflectors) as a diffraction grating is known as one of the wavelength-tunable semiconductor lasers having a conventional multiple-electrode DBR structure.
FIG. 11 is a schematic diagram showing the structure of a conventional SSG-DBR wavelength-tunable semiconductor laser reported by H. Ishii et. al. in IEEE Journal of Quantum Electronics, vol. 32, No. 3, 1996, pp 433-441, and, more particularly, FIG. 11 shows a cross-sectional view of a wavelength-tunable semiconductor laser in the direction parallel to the optical axis.
In FIG. 11, reference numeral 1 represents an active region; 2, a front light reflection area; 3, a rear light reflection area; 4, a phase control area; 5, an optical waveguide formed of InGaAsP; 6, an n-type InP substrate; 7, an n-type InP cladding layer; 8, a p-type InP cladding layer; 9, a p-type InGaAsP contact layer; 13, an n-type electrode; 14a, 14b, 14c, 14d, p-type electrodes; 15, a laser beam emitted from the front facet of an optical resonator; and 21, 22, the pitch variations of the diffraction grating (i.e., one period of modulation) of each of a front SSG-DBR mirror and a rear SSG-DBR mirror, respectively.
Here, the SSG-DBR mirror has the following periodic structure. That is, when in a structure having both the ends spaced at a predetermined distance, the pitch of the diffraction grating in the structure is linearly and continuously varied (linearly chirped) from xcex9a to xcex9b in the direction from one end to the other end, and the structure is set as one period xcex9s, the above structure of one period xcex9s is repeated by plural periods (xcex9s) to thereby achieve the periodic structure. The reflection peak spectrum of the SSG-DBR mirror has plural reflection peaks at a wavelength interval of xcex4xcex=xcex02/(2neqXxcex9s) over the wavelength range from xcexa=2neqXxcex9a to xcexb=2neqXxcex9b. Here, neq represents the equivalent refractive index of the optical waveguide, and xcex0 represents the center wavelength.
In the front SSG-DBR mirror constituting the front light reflection area 2 and the rear SSG-DBR mirror constituting the rear light reflection area 3 of the conventional wavelength-tunable semiconductor laser, one period 21 of the front SSG-DBR mirror and one period 22 of the rear SSG-DBR mirror are respectively repeated by plural periods in FIG. 11 (the repetitive arrangement is not shown in FIG. 11). The SSG-DBR wavelength-tunable semiconductor laser is designed so that the wavelength intervals of the reflection peaks of the front and rear SSG-DBR mirrors are slightly different from each other by using a method of varying the distance of the one period 22 with respect to the distance of the one period 21 above-mentioned.
Next, the operation of the conventional SSG-DBR wavelength-tunable semiconductor laser shown in FIG. 11 will be described.
As shown in FIG. 11, the active region 1, the front light reflection area 2, the rear light reflection area 3, and the phase control area 4 are integrated into the optical waveguide 5. P-type electrodes 14a, 14b, 14c, 14d, which are electrically separated from one another, are located in the respective areas. Active-layer current is injected into the active region 1 by applying a forward bias voltage across the p-type electrode 14b opposite the active region 1 and the n-type electrode 13 located at the rear surface side of the semiconductor substrate, whereby spontaneous emission light having a broad wavelength range is emitted.
The spontaneous emission light is repetitively reflected and amplified by the front SSG-DBR mirror located in the front light reflection area 2 and the rear SSG-DBR mirror located in the rear light reflection region 3 while it propagates in the optical waveguide 5 located in the optical resonator, and laser beams having one wavelength are controlled to be finally selected by controlling refractive index of each area due to current injection to the front light reflection area 2 and/or the rear light reflection area 3 and the phase control area 4, whereby laser oscillation at a single wavelength is achieved with a threshold current.
The laser oscillation wavelength control of the conventional wavelength-tunable semiconductor laser will be described in more detail.
FIG. 12A shows reflection peak spectra of the front SSG-DBR mirror and the rear SSG-DBR mirror formed in the front light reflection area 2 and the rear light reflection area 3 when no current is injected into these areas 2 and 3. FIG. 12B shows the comparison between the reflection peak spectrum of the rear SSG-DBR mirror when current is injected into the rear light reflection area 3 and the reflection peak spectrum of the front SSG-DBR mirror when no current is injected into the front light reflection area 2. In FIGS. 12A and 12B, the abscissa represents the wavelength and the ordinate represents the reflectivity. Further, xcex1 represents the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident when no current is injected into each of the front light reflection area 2 and the rear light reflection area 3, and xcex2 represents the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident when current is injected into the rear light reflection area 3. These reflection peak spectra comprise plural reflection peaks that are different in intensity from each other and have extremely narrow line widths, which is the general characteristic of the SSG-DBR wavelength-tunable semiconductor laser.
As described above, in the initial state when both the front SSG-DBR mirror control current and the rear SSG-DBR mirror control current are equal to zero, the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors in the front light reflection area 2 and the rear light reflection area 3 respectively are coincident with each other is equal to xcex1. As a result, the light having the wavelength xcex1 is strongly reflected from the front and rear SSG-DBR mirrors, so that the loss of light beams at the wavelength xcex1 is much smaller than light beams having the other wavelengths. That is, the gain of light at the wavelength xcex1 is relatively increased as compared with the gain of light at the other wavelengths, so that the wavelength-tunable semiconductor laser starts laser oscillation at the wavelength xcex1. The reason why the reflection peaks of the front and rear SSG-DBR mirrors are coincident with each other at only the wavelength xcex1 and the other reflection peaks at the other wavelengths are not coincident with each other resides in that a minute displacement occurs in the wavelength interval of the reflection peak spectrum between the front and rear mirrors due to the difference in pitch of the diffraction grating which is caused by the difference in distance between the respective periods 21 and 22 and thus the reflection peaks are coincident with each other at only a specific place, like graduations of a vernier.
In order to tune the laser oscillation wavelength of the SSG-DBR wavelength-tunable semiconductor laser, a forward bias voltage is applied to any one of or both the front light reflection area 2 and the rear light reflection area 3 to inject current into the area(s), thereby equivalently varying the refractive index of the front light reflection area 2 and/or the rear light reflection area 3 through the current injection as shown in FIG. 12B. When the refractive index is varied by the current injection, the wavelength at which the gain is relatively larger is shifted to the short wavelength side. The light having this wavelength is amplified while propagating through the optical waveguide 5, and finally induces laser oscillation at the wavelength xcex2 at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident with each other. By using this method, that is, by injecting current into the light reflection area having the SSG-DBR mirror formed therein and controlling the current injection level to intentionally vary the refractive index of the light reflection area, the laser oscillation wavelength of the wavelength-tunable semiconductor laser can be tuned with high controllability. The advantage of the SSG-DBR mirror resides in that each reflection peak intensity is relatively high. Particularly, this effect is more remarkable to an SG-DBR mirror described later.
A sampled grating DBR, that is, SG-DBR reported by V. Jayaraman et al. in IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp. 1824-1834, is known as a mirror for a wavelength-tunable semiconductor laser which is similar to SSG-DBR. Here, SG-DBR means the following repetitive structure. That is, when a portion comprising a diffraction grating portion and a non-diffraction grating portion is set as one period, this structure is repeated by plural periods to thereby achieve a repetitive structure. The diffraction grating of the diffraction grating portion has an ordinary uniform pitch.
FIG. 13 shows the sectional structure of an SG-DBR wavelength-tunable semiconductor laser.
The SG-DBR wavelength-tunable semiconductor laser is different from the SSG-DBR wavelength-tunable semiconductor laser described above only in that the mirrors constituting the front and rear light reflection areas 2 and 3 are SG-DBR mirrors. The characteristic of the SG-DBR mirror resides in that since a diffraction grating portion and a non-diffraction grating portion is set as one period and this paired structure is repeated over plural periods to form the light reflection area, periodic reflection peaks occur in the reflection peak spectrum. In a light reflection area comprising only a diffraction grating having an ordinary uniform pitch, that is, in a DBR mirror, only one reflection peak occurs. Both the SG-DBR mirror and the DBR mirror are remarkably different in this point. However, in the SG-DBR mirror, each reflection peak intensity is not so high, and the respective reflection peaks are different in intensity. Specifically, the reflection peak spectrum of the SG-DBR mirror exhibits a spectral shape that is monotonically reduced in the direction from the center reflection peak to both the sides.
FIG. 12 shows the wavelength-dependence of the reflection peaks of the front and rear SSG-DBR mirrors in the conventional SSG-DBR wavelength-tunable semiconductor laser. As described above, the SSG-DBR wavelength-tunable semiconductor laser is designed so that the wavelength intervals of the reflection peaks of the front and rear SSG-DBR mirrors are made mutually slightly different from each other by using the method of varying the distance of the one period 22 of the rear light reflection area 3 with respect to the distance of the one period 21 of the front light reflection area 2 or the like. That is, the laser oscillation wavelength is varied by using the method of injecting current into the front and rear SSG-DBR mirror areas, that is, the front light reflection area 2 and the rear light reflection area 3 to reduce the refractive index and shift the reflection peaks to the short wavelength side, thereby tuning the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident with each other, that is, by using the method utilizing a so-called vernier effect.
However, the reflection peak intensity randomly varies every SSG mode, and thus when the laser oscillation wavelength is tuned by using the vernier effect as described above, an inter-mode competition against the laser oscillation in another SSG mode which is not originally intended may occur. Accordingly, when the injection current (if) to the front SSG-DBR mirror and the injection current (ir) to the rear SSG-DBR mirror are varied, the laser oscillation wavelength may irregularly vary due to variation of the temperature of the device or the injection current, with high probability. Further, the random variation of the reflection peak intensity induces such a problem that it is difficult to tune the laser oscillation wavelength continuously in some wavelength range.
In the conventional SSG-DBR wavelength-tunable semiconductor laser, the uniformity of the reflection peak intensity can be enhanced by increasing the repetitive period of the one period 21, 22 of the diffraction grating constituting the SSG-DBR mirror to sufficiently increase the reflectivity. In this case, however, if the reflectivity of the front SSG-DBR mirror is set to a high value, there would occur such a problem that the laser beam output which can be taken out to the outside is reduced, and the differential quantum efficiency is also reduced. Further, if the front light reflection area 2 is lengthened, there would newly occur such a problem that the laser oscillation line width is broadened due to a free-carrier absorption effect or carrier recombination effect when the current injection is carried out.
When the front and rear light reflection areas are SG-DBR mirrors, high reflection peak intensity can not be achieved for each reflection peak as compared with the SSG-DBR mirror structure. In addition, the reflection peak intensity is monotonically reduced from the center reflection peak to both the sides, and thus the reflection peak spectrum is not flat over the overall wavelength range, so that there is a problem that the wavelength-tunable range is narrower than that of the SSG-DBR mirror.
Further, in order to transmit a laser beam to a farther place through an optical fiber by using a wavelength-tunable semiconductor laser, it has been practically preferable that maximum light output power is achieved to overcome the optical loss in the optical fiber.
The present invention has been implemented to overcome the problems occurring in the conventional wavelength-tunable semiconductor laser, and has an object to provide a wavelength-tunable semiconductor laser having wavelength stability when the wavelength is tuned, and excellent device characteristics such as low threshold current, high optical output power operation, etc. Further, another object of the present invention is to provide an optical module having excellent wavelength stability and excellent characteristics such as high optical output power, etc.
In order to attain the above objects, according to an aspect of the present invention, the wavelength-tunable semiconductor laser includes: a semiconductor substrate; an optical waveguide provided over the upper surface of the semiconductor substrate; a front light reflection area that is provided at the front side in a laser beam emission direction by a part of the optical waveguide and comprises an SG-DBR mirror achieved by structurally repeating a portion comprising a pair of diffraction grating portion and a non-diffraction grating portion over plural periods on the assumption that the portion is set as one period; a rear light reflection area that is provided at the rear side in the laser beam emission direction by a part of the optical waveguide and comprises an SSG-DBR mirror achieved by structurally repeating, over plural periods, a portion in which the pitch of a diffraction grating pitch regularly varies from one end to the other end, both the ends being spaced at a predetermined distance, on the assumption that the portion is set as one period; an active region comprising an active layer provided between the front light reflection area and the rear light reflection area by a part of the optical waveguide; and a phase control area that is provided between the front light reflection area and the rear light reflection area by a part of the optical waveguide, and comprises a phase control layer whose refractive index is varied by current injection. Accordingly, the wavelength-tunable semiconductor laser has excellent wavelength stability when the wavelength is tuned.
According to another aspect of the present invention, the optical module includes: a wavelength-tunable semiconductor laser including a semiconductor substrate, an optical waveguide provided over the upper surface of the semiconductor substrate, a front light reflection area that is provided at the front side in a laser beam emission direction by a part of the optical waveguide and comprises an SG-DBR mirror achieved by structurally repeating a portion comprising a pair of diffraction grating portion and a non-diffraction grating portion over plural periods on the assumption that the portion is set as one period, a rear light reflection area that is provided at the rear side in the laser beam emission direction by a part of the optical waveguide and comprises an SSG-DBR mirror achieved by structurally repeating, over plural periods, a portion in which the pitch of a diffraction grating pitch regularly varies from one end to the other end, both the ends being spaced at a predetermined distance, on the assumption that the portion is set as one period, an active region comprising an active layer provided between the front light reflection area and the rear light reflection area by a part of the optical waveguide, and a phase control area that is provided between the front light reflection area and the rear light reflection area by a part of the optical waveguide, and comprises a phase control layer whose refractive index is varied by current injection; a focusing lens for focusing laser beams emitted from the wavelength-tunable semiconductor laser; an optical fiber for guiding the laser beams thus focused, and a housing in which the wavelength-tunable semiconductor laser, the focusing lens and the optical fiber are fixed. Accordingly, the laser beams having excellent wavelength stability can be achieved as an optical fiber output.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.